A capacitor is designed to hold a large amount of charge and withstand the voltage that appears across its dielectric. If the capacitor is designed to store charge for a long period of time and discharge in a short period of time, in the order of .mu.sec, it is termed an energy discharge capacitor. Typically these capacitors are used by charging them in parallel across a voltage and discharging them in series. Such banks of capacitors are used to produce a high voltage pulse of electricity.
One type of energy discharge capacitors is termed self-healing since it can still function after undergoing dielectric breakdown. These capacitors are constructed of a thin sheet of dielectric such as polypropylene which is coated on both surfaces with a thin (300 Angstroms) layer of aluminum. Each of the coated surfaces of the dielectric acts like a metal plate. Typically the dielectric sheet is three inches wide and 600 ft. long, providing a very large area for the accumulation of charge. Each surface has an electrical contact attached along an entire long edge of the surface. The thin sheet is wrapped so that it can be immersed in a relatively small oil filled cannister. The oil helps remove the heat generated during charging and discharging.
These capacitors are self-healing because once a dielectric breakdown occurs, there is a surge of current through the dielectric in the region of breakdown. This instantly causes a vaporization and oxidation of the aluminium in the region of the breakdown. The oxidation of the aluminium to alumina oxide creates an insulator which prevents further current flow in the region of damage. This vaporization and oxidation is termed clearing. Generally the clearing is restricted to a 1/16 in. diameter area around the breakdown site. With the area cleared the capacitor can then resume storing charge although with less capacitance due to the loss of a portion of the metalized material.
Since it is generally desirable to make capacitors with the ability to hold more charge or be charged to a higher voltage than is possible using a single sheet of metalized dielectric, capacitor manufacturers typically place more than one of these wrapped metalized dielectric sheets together in a single cannister. Each wrapped metalized dielectric sheet is termed a section. Sections connected in series result in the total voltage produced across the series being the sum of the voltages across the individual sections. Sections connected in parallel experience the same voltage but result in more charge being stored by the capacitor.
FIG. 1 depicts a typical multi-section capacitor. Sections connected in parallel, such as 10a and 10b each experiences a voltage across it of (V.sub.i). The sections connected in series such as 10a, 12a, 14a experience voltages of (V.sub.i, V.sub.j, V.sub.k), respectively, and the voltage across the entire series is (V.sub.T), which is the sum of the voltages (V.sub.i, V.sub.j, V.sub.k) of the individual sections 10, 12, 14. This total voltage (V.sub.T) appears across the terminals of the capacitor 20, 22.
FIG. 2 depicts the voltage V across each individual section as a function of time. Referring to FIGS. 1 and 2, initially all sections are at zero voltage. At time T.sub.1 the capacitor begins to charge and its voltage 106 increases until time T.sub.2 when the capacitor reaches its working voltage (V.sub.T) and the sections reach their operating voltage 108. At this time the operating voltage across each section is about V.sub.0. At time T.sub.2 a discharge 112 occurs and the voltage returns zero. At time T.sub.3 charging begins again 116, but before the operating voltage (V.sub.0) of the sections is reached, at time T.sub.4, a dielectric breakdown 120 occurs one of the sections; in this example 10(b) of FIG. 1. Some of the charge across the capacitor sections 10(a) and 10(b) will recombine because of the failure of 10(b) and the resulting clearing process. This will cause the voltage (V.sub.i) across these sections to drop, but the voltages across the other series sections (V.sub.j, V.sub.k) will not be effected. The circuit charging the capacitor is usually constructed to charge the capacitor to the designated operating voltage (V.sub.T). When the capacitor is fully charged to (V.sub.T), the voltage (V.sub.i) across the section 10 that cleared will be less than normal due to the damage caused by the clearing. However, the voltage (V.sub.j, V.sub.k) across the other sections 12, 14 will be higher since the voltage across the capacitor (V.sub.T) is the summation of (V.sub.i, V.sub.j, V.sub.k). At time T.sub.5 the capacitor is discharged 130 but because of a shifting of charge among the sections the voltage (V.sub.i) goes 134 well below the zero potential, while the voltages (V.sub.j, V.sub.k) of the undamaged sections 12, 14 stay 136 above zero. This is true even though the voltage across the capacitor is now zero.
At time T.sub.6, charging again resumes until time T.sub.7 when a discharge 144 again occurs. The peak voltage 138 on the undamaged sections 12, 14 again exceeds the operating voltage (V.sub.0) while the voltage across the damage section is less 140. At T.sub.8 charging again occurs but before the charging is completed, another breakdown and clearing takes place 154 at time T.sub.10.
Because this breakdown has taken place in a section, for example 12a, which was above the operating voltage (V.sub.0), the damage and clearing was greater than in the previous breakdown 120. So the peak voltage 158 that section 12 will reach is less than the voltage 156 that the previously damaged section 10 reached at peak. Again, the charging circuit, in order to bring the voltage across the capacitor to the operating voltage (V.sub.T), raises the voltage on the undamaged sections 160 above their normal operating voltage (V.sub.0).
When the discharge occurs at T.sub.11, the voltage 164 on the more damaged section 12 goes below zero, while the voltage on the previously damaged section 10 stays above 166, as does the voltage 168 on the undamaged section 14. At T.sub.12 charging again occurs with the voltage 176 on the section 12 with the most damage remaining below the operating point (V.sub.0) and the voltage 172, 174 on the others 10, 14 being forced above their normal operating point (V.sub.0) by the charging circuit.
Because some sections are operating above the normal voltage for that section, that section is a candidate for failure. As more sections experience breakdown, the damage accumulates until the capacitor fails. The cause of this failure is that while the sections start with the same voltages across them, failures cause potential differences to appear between the sections. This in turn causes further failures and other potential differences to appear between sections. To avoid the total failure of a capacitor, it must be configured so as to remove the potential differences which arise between sections.